Temporal variation in selection on male and female traits in wild tree crickets.

Kyla Ercit

doi:10.1002/ece3.2105

Abstract

Understanding temporal variation in selection in natural populations is necessary to accurately estimate rates of divergence and macroevolutionary processes. Temporal variation in the strength and direction of selection on sex‐specific traits can also explain stasis in male and female phenotype and sexual dimorphism. I investigated changes in strength and form of viability selection (via predation by wasps) in a natural population of male and female tree crickets over 4 years. I found that although the source of viability stayed the same, viability selection affected males and females differently, and the strength, direction and form of selection varied considerably from year to year. In general, males experienced significant linear selection and significant selection differentials more frequently than females, and different male traits experienced significant linear selection each year. This yearly variation resulted in overall weak but significant convex selection on a composite male trait that mostly represented leg size and wing width. Significant selection on female phenotype was uncommon, but when it was detected, it was invariably nonlinear. Significant concave selection on traits representing female body size was observed in some years, as the largest and smallest females were preyed on less (the largest may have been too heavy for flying wasps to carry). Viability selection was significantly different between males and females in 2 of 4 years. Although viability selection via predation has the potential to drive phenotypic change and sexual dimorphism, temporal variation in selection may maintain stasis.

Highlights

Understanding the pace of phenotypic selection in natural populations is important to accurately calculate the rates of divergence and macroevolutionary processes, and to estimate how quickly populations can respond to change, man-made or otherwise (Siepielski et al 2009)
Cubic spline analysis showed that the form of viability selection on principal component axis 1 was considerably different from year to year among both males and females (Fig. 1)
Males were subject to significant directional selection within years, but which traits were under selection changed significantly from year to year, resulting in no significant selection gradients on any one trait over the 4-year period

Summary

Introduction

Understanding the pace of phenotypic selection in natural populations is important to accurately calculate the rates of divergence and macroevolutionary processes, and to estimate how quickly populations can respond to change, man-made or otherwise (Siepielski et al 2009). Compiled estimates of selection predict faster microevolution than is generally observed (Kinnison and Hendry 2001). This mismatch could be due to overestimating selection in the long term (over several generations) when using selection gradients measured in short term (over a single generation or single breeding season), as estimates of selection can be greatly affected by the duration of the episode of selection (Hoekstra et al 2001). Longer-term studies of selection on natural populations are valuable as they provide information on how the strength and direction of selection is affected by timescale (Kingsolver et al 2001; Kingsolver and Pfennig 2007). In longer intervals of time, such as over several generations, temporal variation in the strength and direction of selection and variation in which traits are under selection can reduce the magnitude of net selection across years, which would result in slower evolution than predicted from a single episode of selection (Siepielski et al 2009, 2011; and see Morrissey and Hadfield 2012)

Methods

Results

Conclusion